An optical cable for communications typically includes a number of optical fibers. The optical fibers housed in an optical cable may be arranged in one or more bundles, each bundle of optical fibers being housed in a respective retaining element. The bundle of optical fibers with the respective retaining element is typically termed “micromodule.”
A micromodule typically includes 2 to 12 optical fibers arranged substantially parallel to the axis of the micromodule or according to an open helix pattern (generally termed “S-Z”) about the axis of the micromodule. The retaining element is generally made of a polymeric material, in particular a thermoplastic material optionally charged with mineral fillers. The space between the optical fibers within the retaining element may be empty, or it may be filled with jelly, talc, or water-swellable yarns for preventing propagation of water within the micromodule.
The one or more micromodules form the so-called “optical core” of the optical cable. The optical core is generally inserted into a protective sheath that is also made of a polymeric material. The protective sheath typically has a number of elongated reinforcing elements arranged in the thickness thereof and parallel to the axis of the optical cable.
U.S. Pat. No. 5,155,789, which is hereby incorporated by reference in its entirety, discloses a telecommunications cable comprising a series of optical fibers split into modules, each of which is enveloped by a thin supporting sheath, the sheaths being in contact with the optical fibers. The supporting sheath is made of a plastic, such as polyethylene, polypropylene, or polyamides. The supporting sheath is constituted by a layer of thickness lying in the range a few thousandths of a millimeter to a few tenths of a millimeter and preferably lying in the range one hundredth of a millimeter to one tenth of a millimeter. The supporting sheath material may extend between the fibers to obtain a more intimate contact, and indeed the optical fibers may be completely embedded in the material constituting the supporting sheath.
U.S. Pat. No. 5,671,312, which is hereby incorporated by reference in its entirety, discloses a cable comprising a series of optical fibers, each of which is covered in a primary sheath. The optical fibers are collected together in modules that may contain some variable number of optical fibers. Each module is itself wrapped in a thin supporting sheath that is easily tearable and that is intended to keep the optical fibers in contact with one another so as to provide mechanical coupling between them.
U.S. Pat. No. 6,334,015, which is hereby incorporated by reference in its entirety, discloses a telecommunication cable including a plurality of optical fibers. A thin and generally cylindrical retaining sheath envelops the optical fibers. The retaining sheath tightly grips a predetermined number N of optical fibers to hold the optical fibers in a group and thereby constitute a compact module. The retaining sheath is extruded into a thermoplastics material. The material of the retaining sheath has a thickness less than 0.3 mm, preferably in the range from 0.1 to 0.2 mm. For example, the material of the retaining sheath is an amorphous thermoplastics material for example polyvinylchloride (PVC) or an elastomer; or a charged thermoplastics material, for example polyethylene or a polyolefin such as ethylene vinyl acetate (EVA), containing a sufficient quantity of one or more mineral charges.
U.S. Pat. No. 6,937,802, which is hereby incorporated by reference in its entirety, discloses a telecommunication cable including a plurality of modules, each with a thin retaining sheath for clamping optical fibers together. Each retaining sheath contains plural respective modules and is mechanically coupled to the retaining sheaths of the respective modules to form super-modules that contact an outside jacket. The thickness of the sheath is at most about a few tenths of a millimeter, typically from 0.1 mm to 0.5 mm.
The above listed documents teach optical fibers contained in modules having a thin sheath. The modules houses the optical fibers as tightly as possible to ensure the coupling (or clamping) thereof.
International Publication No. WO 01/21706, which is hereby incorporated by reference in its entirety, discloses a material for forming thin films, in particular for the outer sheath of a micromodule for an optical cable. The material includes a composition containing an olefin polymer and a filler amount ranging between 25 and 65 weight percent (wt %) of the composition, said material in an undivided state having a tensile strength ranging between 6 and 20 MPa and an elongation at break ranging between 50% and 300%.
U.S. Pat. No. 7,082,241, which is hereby incorporated by reference in its entirety, discloses a telecommunication cable of the microcable or minicable type having optical fibers contained in a thin retaining sheath. The retaining sheath clamps the optical fibers to hold the optical fibers in groups. The retaining sheath is in contact with and mechanically coupled to the optical fibers. The coupling between the optical fibers and the retaining sheath referred to above is defined as a mechanical coupling between two members, meaning that any stress applied to one of the members is transferred to the other member or that if one of the members is stressed the other is also stressed, without this necessitating any bonding or other fixing of one of the members to the other. The retaining sheath has a thickness of the order of a few tenths of a millimeter, typically 0.25 mm. The material of the retaining sheath is an amorphous thermoplastic material, an elastomer, or a thermoplastic material that can contain mineral charges.
U.S. Pat. No. 6,658,184, which is hereby incorporated by reference in its entirety, discloses a telecommunications cable module comprising a plurality of optical fibers. The optical fibers are surrounded by a flexible tube referred to as a “skin.” The thickness of the skin lays in the range 0.05 mm to 0.25 mm, and is preferably 0.15 mm. The skin is of a thermoplastic elastomer having flexible diol segments, with a melting point greater than 130° C. and initial tearing strength. The material disclosed preferably presents breaking elongation lying in the range 50% to 300% and tensile strength lying in the range 5 MPa to 15 MPa. The material has hardness on the Shore D scale of less than 50. The elastomer properties are provided by flexible segments in the polymer chain (copolymers with polyether segments).
U.S. Pat. No. 6,215,931, which is hereby incorporated by reference in its entirety, discloses a bundle of optical fibers surrounded by a buffer tube in near-tight configuration. The buffer tube is made from a thermoplastic polyolefin elastomer having a room temperature modulus of elasticity below about 500 MPa and a low temperature (−40° C.) modulus of elasticity below about 1500 MPa. The buffer material has a Melt Flow Index above about 3. The thermoplastic polyolefin elastomer material used to form the buffer tube may also contain organic or inorganic fillers. The thermoplastic polyolefin elastomer material forming the buffer tube has an elongation at break <500% at room temperature. If the modulus of elasticity and elongation at break are low enough, the buffer tube can be easily removed without special tools and without damaging the optical fiber or fibers disposed therein.
International Publication No. WO 2006/034722 (and its counterpart U.S. Pat. No. 7,536,071), which are hereby incorporated by reference in their entirety, disclose a micromodule comprising a plurality of optical fibers and a retaining element for containing the optical fibers. The retaining element of the micromodule is made of a thermoplastic polymeric composition having an elastic module lower than 500 MPa at 20° C., an ultimate tensile strength comprised between 5 and 10 MPa, and an elongation at break comprised between 30% and 80%. The retaining element has a thickness that is preferably between 0.1 and 0.2 mm. In the case of a micromodule containing 12 optical fibers, the micromodule has an overall size having a maximum transversal dimension comprised between 1.25 mm and 1.45 mm.